System and method for return electrode monitoring

ABSTRACT

A system for determining probability of tissue damage is disclosed. The system includes an electrosurgical generator adapted to generate an electrosurgical current and a plurality of return electrodes adhered to a patient and adapted to couple to the electrosurgical generator. Each of the return electrodes includes an impedance sensor attached thereto. The system also includes a current monitor connected in series with each of the plurality of the return electrodes to measure the electrosurgical current passing therethrough and a processor coupled to each of the current monitors. The processor is configured to calculate a cooling factor and a heating factor for each of the plurality of the return electrodes. The processor further configured to determine probability of tissue damage for each of the plurality of the return electrodes as a function of the cooling factor and the heating factor.

BACKGROUND

1. Technical Field

The present disclosure relates generally to a system and method forusing a plurality of return electrodes during electrosurgery and, moreparticularly, to a system and method for balancing the various thermaleffects of the plurality of return electrodes by minimizing theprobability of tissue damage and ensuring the plurality of returnelectrodes are properly attached to a patient.

2. Background of Related Art

During electrosurgery, a source or active electrode delivers energy,such as radio frequency (RF) energy, from an electrosurgical generatorto a patient and a return electrode or a plurality thereof carry currentback to the electrosurgical generator. In monopolar electrosurgery, thesource electrode is typically a hand-held instrument placed by the userat the surgical site and the high current density flow at this electrodecreates the desired surgical effect of ablating, cutting or coagulatingtissue. The patient return electrodes are placed at a remote site fromthe source electrode and are typically in the form of pads adhesivelyadhered to the patient.

The return electrodes usually have a large patient contact surface areato minimize heating at that site since the smaller the surface area, thegreater the current density and the greater the intensity of the heat.That is, the area of the return electrode that is adhered to the patientis important because it is the current density of the electrical signalthat heats the tissue. A larger surface contact area is desirable toreduce heat intensity. Return electrodes are sized based on assumptionsof the maximum current seen in surgery and the duty cycle (i.e., thepercentage of time the generator is on) during the procedure.

The first types of return electrodes were in the form of large metalplates covered with conductive jelly. Later, adhesive electrodes weredeveloped with a single metal foil covered with conductive jelly orconductive adhesive. However, one problem with these adhesive electrodeswas that if a portion peeled from the patient, the contact area of theelectrode with the patient decreased, thereby increasing the currentdensity at the adhered portion and, in turn, increasing the heat appliedto the tissue. This risked burning the patient in the area under theadhered portion of the return electrode if the tissue was heated beyondthe point where circulation of blood could cool the skin.

To address this problem, split return electrodes and hardware circuits,generically called return electrode contact quality monitors (RECQMs),were developed. These split electrodes consist of two separateconductive foils. The hardware circuit uses an AC signal between the twoelectrode halves to measure the impedance therebetween. This impedancemeasurement is indicative of how well the return electrode is adhered tothe patient since the impedance between the two halves is directlyrelated to the area of patient contact with the return electrode. Thatis, if the electrode begins to peel from the patient, the impedanceincreases since the contact area of the electrode decreases. CurrentRECQMs are designed to sense this change in impedance so that when thepercentage increase in impedance exceeds a predetermined value or themeasured impedance exceeds a threshold level, the electrosurgicalgenerator is shut down to reduce the chances of burning the patient.

Currently, during electrosurgical procedures involving especially highcurrent, it is common to use multiple return electrodes to ensureadequate surface area to minimize heating at the return electrodes andthereby minimize the risk of damaging tissue. Typical ablationprocedures can deliver up to 2.0 A_(rms) for up to 20 minutes eithercontinuously or with periodic current pulses. This extended duration fora high output current value may create a potential for alternate siteburns due to return electrode pad heating. Further, the use of multiplereturn electrodes may also pose an additional potential problem—theincrease in temperature under each of the return electrodes is notuniform, e.g., there is a thermal imbalance among the multiple returnelectrodes. This is caused by the differing impedance values between theactive electrode and each of the multiple return electrodes, whichvaries due to placement and proximity of the active electrode to thereturn electrode.

Typically, since current is the primary factor in return electrodeheating, measurement of the output current from the electrosurgicalgenerator may be used to infer possible tissue damage. Although theoutput current of the electrosurgery generator is approximately equal tothe sum of the current through each of the return electrodes, theindividual return electrode currents may not be equal due to thediffering impedances as described above. This condition may generate animbalance of current among each of the return electrodes resulting in animbalance of thermal rise on the return electrodes.

SUMMARY

Systems and methods for ensuring the plurality of return electrodes areproperly attached to a patient, balancing thermal effects, and reducingprobability of tissue damage during electrosurgical procedures involvinga multitude of return electrodes are disclosed. More specifically, thesystem includes an electrosurgical generator and a plurality of returnelectrodes as well as a current monitor electrically connected to eachof the return electrodes and the electrosurgical generator. Thegenerator monitors the current passing through each of the returnelectrodes through the current monitor. The generator determines currentload ratio for each return electrode to ensure the return electrodes areproperly attached to patient.

According to one embodiment of the present disclosure, a system fordetermining probability of tissue damage is disclosed. The systemincludes an electrosurgical generator adapted to generate anelectrosurgical current and a plurality of return electrodes adhered toa patient and adapted to couple to the electrosurgical generator. Eachof the return electrodes includes an impedance sensor attached thereto.The system also includes a current monitor connected in series with eachof the plurality of the return electrodes to measure the electrosurgicalcurrent passing therethrough and a processor coupled to each of thecurrent monitors. The processor is configured to calculate a coolingfactor and a heating factor for each of the plurality of the returnelectrodes. The processor is further configured to determine probabilityof tissue damage for each of the plurality of the return electrodes as afunction of the cooling factor and the heating factor.

According to another embodiment of the present disclosure, a method fordetermining probability of tissue damage is disclosed. The methodincludes the step of providing a plurality of return electrodes adheredto a patient and adapted to couple to an electrosurgical generatorconfigured to generate an electrosurgical current, wherein each of thereturn electrodes includes an impedance sensor and a current monitorconnected in series with each of the plurality of the return electrodes.The method also includes the steps of measuring the electrosurgicalcurrent passing through each of a plurality of the return electrodes andmeasuring the impedance of each of the plurality of the returnelectrodes. The method further includes the steps of calculating aheating factor adjacent the return electrode for each of the pluralityof the return electrodes, calculating a cooling factor adjacent thereturn electrode for each of the plurality of the return electrodes anddetermining probability of tissue damage for each of the plurality ofthe return electrodes as a function of at least one of the coolingfactor and the heating factor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of a monopolar electrosurgicalsystem;

FIG. 2 is a schematic block diagram of an electrosurgical system fordetermining adherence of multiple return electrodes to a patient;

FIG. 3 is a flow diagram showing a method for determining adherence ofmultiple return electrodes to a patient;

FIG. 4 is a schematic block diagram of on electrosurgical system fordetermining the probability of tissue damage and controlling returncurrent in multiple return electrodes;

FIG. 5 is a flow diagram showing a method for monitoring and controllingreturn electrode current in multiple return electrodes; and

FIG. 6 is a flow diagram showing a method for determining theprobability of tissue damage.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described hereinbelow withreference to the accompanying drawings. In the following description,well-known functions or constructions are not described in detail toavoid obscuring the present disclosure in unnecessary detail.

Systems and methods for determining whether return electrodes areproperly attached to a patient, for balancing thermal effects ofmultiple return electrodes, and for preventing tissue damage when usingmultiple return electrodes are disclosed.

FIG. 1 is a schematic illustration of a monopolar electrosurgical system1. The system 1 includes a surgical instrument 11, e.g., an activeelectrode, for treating tissue at a surgical site. Electrosurgicalenergy is supplied to the instrument 11 by a generator 10 via a cable 18allowing the instrument 11 to ablate, cut or coagulate the tissue. Theelectrosurgical system also includes a plurality of return electrodes14, 15 and 16 placed under the patient's back, the patient's leg, andthe patient's arm, respectively, to return the energy from the patientto the generator 10 via a cable 12. The return electrodes 14, 15 and 16are preferably in the form of a split pad which is adhesively attachedto the patient's skin.

The surface area of the return electrodes 14, 15 and 16 that adheres tothe patient is substantially similar since the surface area affects thecurrent density of the signal which, in turn, heats the tissue. Thesmaller the contact area of the return electrode with the patient'stissue, the greater the current density and concentrated heating oftissue underneath the return electrodes 14, 15 and 16. Conversely, thegreater the contact area of the return electrodes 14, 15 and 16, thesmaller the current density and the less heating of tissue.

FIG. 2 illustrates a schematic block diagram of the electrosurgicalsystem 1 for determining whether the return electrodes 14, 15 and 16 areproperly adhered to the patient's body. The system 1 includes generator10 for generating electrosurgical energy, an active electrode 11, e.g.,an instrument, for delivering the electrosurgical energy to the tissueand a plurality of return electrodes 14, 15 and 16 for receiving theelectrosurgical energy and returning the electrosurgical energy to thegenerator 10. Although the present disclosure describes theelectrosurgical system 1 in reference to three return electrodes 14, 15and 16, those skilled in the relevant art will understand that theprinciples of the present disclosure may be used with any number ofreturn electrodes. In one embodiment, the system measures impedancebetween a pair of split pads of the return electrode via an impedancesensor to determine adherence of the return electrode to the patient

The generator 10 includes a microprocessor 26, an adjustable powersupply 22, such as a high voltage supply coupled to an RF output stage24 which generates RF energy for transmission to the instrument 11. Themicroprocessor 26 includes a plurality of input ports. A first inputport in electrical communication with an output current sensor 28measures the output current (I_(OUTPUT)) being transmitted to thepatient through the instrument 11.

The return electrodes 14, 15 and 16 are electrically connected to thegenerator 10 through the cable 12 and in series to current monitors 43,53 and 63, which are connected to the microprocessor 26 and report thecurrent passing through the respective return electrodes 14, 15 and 16.When using multiple return electrodes, monitoring the output current bythe generator 10 may not be accurate in measuring the current passingthrough each of the return electrodes 14, 15 and 16. Therefore, thesystem according to the present disclosure places the current monitors43, 53 and 63 in series with the corresponding return electrodes 14, 15and 16 to allow accurate current measurements to measure current passingthrough each of the return electrodes, I_(Mx), where x is the number ofthe return electrode.

FIG. 3 shows a method for determining adherence of multiple returnelectrode pads to a patient. This is accomplished by determining theratios of the current load on each of the return electrodes 14, 15 and16. Ideally, the return electrodes 14, 15 and 16 are of the same sizeand made from the same material. In absence of interference from othervariables affecting impedance (e.g., temperature, moisture, etc.), thecurrent load on each return electrode is the same since their impedanceis the same. Alternatively, pads of different size or shape can be usedwith adjustment made to the allowable ratio. Current load is determinedby calculating the ratio of the current distribution on each of thereturn electrodes 14, 15 and 16. For instance, if there are three returnelectrodes (e.g., the return electrodes 14, 15 and 16) then the ratio ofthe current load on each of the return electrodes should be 33% (i.e.,total current, I_(TOTAL), divided by the number of returnelectrodes—current passing through each return electrode I_(Mx) is 33%of the total current output). If that ratio changes, it follows that thecurrent load is distributed unevenly. This is caused by differingimpedance of each of the return electrodes or tissue between the activeelectrode and each respective return electrode. Since all of the returnelectrodes are of the same size and material, the differing impedancesare caused by the placements and/or adherence of the return electrodes.Hence, determining the ratios of the current load allows the system todetermine whether the return electrodes 14, 15 and 16 are placedproperly on the patient and are properly adhered thereto.

The presently disclosed process verifies the ratios at two stages:first, prior to commencement of an electrosurgical procedure, andsecond, during the procedure. In step 100, an initial check of adherenceof the return electrodes 14, 15 and 16 is performed. The returnelectrodes 14, 15 and 16 are placed on the patient and the generator 10is activated. The generator 10 outputs a low level interrogation currentto calculate the baseline ratio for each of the return electrodes 14, 15and 16. A low level interrogation current is used since the methodinitially verifies the placement and adherence of the return electrodes14, 15 and 16 prior to commencement of electrosurgery. Current ismeasured by the current monitors 43, 53 and 63 and the measurements aretransmitted to the microprocessor 26.

In step 102, the generator 10 determines the percentage of total currentI_(TOTAL) passing through each return electrode 14, 15 and 16 andcompares the calculated values to the preferred ratio (e.g., 33%). Instep 104, the generator 10 determines if the current load ratios of thereturn electrodes 14, 15 and 16 are equal (i.e., larger or smaller than33%). The ratios may be considered equal if they fall within apredetermined threshold. For instance, if the ratio denoting that thereturn electrode is properly adhered to the patient is 33% and theactual ratio (e.g., 31%) is within a predetermined threshold (e.g., 2%),the two ratios are considered equal. This eliminates the probability ofinsignificant changes in the current load affecting the comparisonprocess.

If the ratios are not equal, then the generator 10 (e.g., microprocessor26) signals that the placement of the return electrodes 14, 15 and 16needs to be checked and adjusted if required in step 106. Afterreadjustment, in step 106, the generator 10 outputs low interrogationcurrent again, to verify that the readjustment corrected the problem.The process loops until the ratios of the current load of each of in thereturn electrodes 14, 15 and 16 are equal or within a predeterminedtolerance.

If the ratios of the current load are equal, then the process continuesto step 108, wherein a second check of the ratios is performed as RFenergy is supplied to the tissue and returned via the return electrodes14, 15 and 16. The current monitors 43, 53 and 63 measure the currentpassing through the return electrodes 14, 15 and 16 throughout theprocedure and transmit the measurements to the generator 10. In step110, the generator 10 again determines the percentage of total currentI_(TOTAL) passing through each return electrode 14, 15 and 16 andcompares the calculated values to the preferred ratio (e.g., 33%).

In step 112, the generator 10 determines if the current load ratios ofthe return electrodes 14, 15 and 16 has changed from the baselinemeasurements taken prior to commencement of the electrosurgicalprocedure by comparing the measured current ratio to the preferredcurrent ratio. If the ratios have changed, the algorithm of thegenerator 10 assumes that the positioning of the return electrodes 14,15 and 16 has also changed since the last check of the ratios in step104. If there is a change, in step 114 the generator 10 adjusts the RFcurrent output or shuts down. The action taken by the generator 10depends on the degree in the change. A relatively small change in theratio (e.g., below 5% for a three return electrode system) may requirean adjustment in the RF energy output. This may be accomplished usingswitches (e.g., switches 44, 54 and 64 shown in FIG. 4 and described inmore detail below). A large change in the ratio (e.g., 5% or more) mayrequire shutting down the generator 10. If the generator 10 shuts down,then the process proceeds to step 106, which is an optional step, whereadjustments to the placement and positioning of the return electrodes14, 15 and 16 are made.

If the ratios are unchanged as determined in step 112, then the processloops to step 108, where the ratio is continually verified during theelectrosurgical procedure. This process ensures that the returnelectrodes 14, 15 and 16, are properly attached to the patient prior toand during electrosurgery, thereby allowing the RF energy to beefficiently dissipated.

FIG. 4 illustrates a schematic block diagram of the electrosurgicalsystem 1 for determining the probability of tissue damage andcontrolling the return current in multiple return electrodes 14, 15 and16. In addition to the components shown in FIG. 2 and described above,the system of FIG. 4 includes switches 44, 54 and 64 and impedancesensors 40, 50 and 60. Further, the generator 10 also includes a current“on” time calculator 30 and a current “off” time calculator 32electrically connected to the microprocessor 26. In embodiments, thecalculators 30 and 32 may be implemented as software applicationsconfigured to be executed by the microprocessor 26.

The “on” time calculator 30 determines the amount of the time thecurrent is being supplied to any one of the multiple return electrodes14, 15 and 16 and transmits this data to the microprocessor 26.Conversely, the “off” time calculator 32 calculates the amount of timethat any one of the return electrodes 14, 15 and 16 did not receive anycurrent or the RF output current was turned “off” and sends a signal tothe microprocessor 26 via one of its input ports.

The return electrodes 14, 15 and 16 are electrically connected in seriesto the current monitors 43, 53 and 63 and the switches 44, 54 and 64,respectively. The current monitors 43, 53 and 63 are connected to themicroprocessor 26 and report the current passing through the respectivereturn electrodes 14, 15 and 16. The switches 44, 54 and 64 areconnected to the time calculators 30 and 32 so that the time calculators30 and 32 can calculate if the return electrodes 14, 15 and 16 areincluded in the circuit. In addition, the switches 44, 54 and 64 areconnected to a controller 25 which regulates whether the switches 44, 54and 64 are open or closed.

The return electrodes 14, 15 and 16 include a pair of split pads 41, 42,51, 52, 61 and 62, respectively, which are electrically connected toimpedance sensors 40, 50 and 60. The function of the sensors 40, 50 and60 will be discussed with reference only to the sensor 40 and itscorresponding components. The sensor 40 measures the impedance betweenthe split pads 41, 42 of the return electrode 14 to determine the degreeof adherence of the return electrode 14. That is, if a portion of thereturn electrode 14 becomes detached from the patient, the impedancewill increase. The sensor 40 transmits a signal indicative of themeasured impedance to an input port of the microprocessor 26. Thoseskilled in the art will appreciate that the return electrodes 14, 15 and16 may include multiple pairs of split pads.

In using multiple return electrodes, monitoring the output currentoutput by the generator 10 is an inaccurate measure of the currentpassing through each of the return electrodes 14, 15 and 16. Therefore,the system according to the present disclosure places the currentmonitors 43, 53 and 63 and the switches 44, 54 and 64 in series with thecorresponding return electrodes 14, 15 and 16. The switches 44, 54 and64 can be active components, such as transistors of various types,(e.g., field effect transistor, insulated gate bipolar transistor, etc.)or electromechanical components (e.g., relays, solenoid switches, etc.).

The return electrodes 14, 15 and 16 are connected to the generator 10through the cable 12. As will be discussed in more detail below, toobtain current measurements for each of the individual return electrodes14, 15 and 16, the current monitors 43, 53 and 63 are included in thecircuit between the return electrodes 14, 15 and 16 and the cable 12.The switches 44, 54 and 64 are also incorporated into the circuit in thesame manner.

Monitoring and controlling of current passing through the returnelectrodes 14, 15 and 16 for balancing thermal effects will be discussedin conjunction with FIG. 5. In step 116, the current passing througheach of the return electrodes 14, 15 and 16 (I_(Mx), wherein x is thenumber of the current monitor) is measured using the respective currentmonitors 43, 53 and 63 and is transmitted to the microprocessor 26.

In step 118, the current passing through all return electrodes 14, 15and 16, I_(TOTAL), is calculated by the microprocessor 26 by summationof current monitor values I_(Mx) for each of the return electrodes. Instep 120, a threshold current value, I_(TH), is calculated by themicroprocessor 26 based on formula (1):

I _(TH) =I _(TOTAL) /n+TOLERANCE.   (1)

In the formula (1), I_(TOTAL) is the value calculated in step 118, n isthe number of return electrodes and TOLERANCE is a predetermined valuerepresentative of the current for any particular return electrodeexceeding the average return electrode current value. TOLERANCE can befrom about 0 mA to about 100 mA. Further, tolerance can also be apercentage of the average current value from about 0% to about 25%.

Once the microprocessor 26 calculates I_(TH), the value is transmittedto the comparator 34. In step 122, all of the I_(Mx) values are comparedto determine the highest return electrode current value I_(high). Thehighest I_(Mx) (e.g., I_(high)) is then sent to the comparator 34.

In step 124, the comparator 34 determines if the current load isunbalanced, for instance, the current passing through the returnelectrode 14, is higher than the current passing through other returnelectrodes 15 and 16. The comparator 34 compares the highest I_(Mx)value with I_(TH) to determine if the highest return electrode currentvalue from step 122 exceeds the predetermined allowable currentthreshold. If the highest return electrode current does not exceed theallowable current threshold, all measured return electrode currents arewithin the allowable tolerance and the process is repeated from step116. Conversely, if the highest return electrode current exceeds theallowable current threshold, then it is expected that the returnelectrode will overheat and possibly damage tissue. In that case, thecomparator 34 notifies the controller 25 of the imbalance in the currentof the return electrode with the highest measured current the processproceeds to step 126.

In step 126, the “off” time for the return electrode having highestI_(Mx), is calculated by the microprocessor 26 using formula (2):

Toff=(Toffmax−Toffmin)/(I _(TOTAL) −I _(TH))² *I _(high) ²   (2)

In formula (2), Toffmax is the maximum off-time period, which is thelongest possible duration of time that a particular return electrode canbe disconnected. Toffmin is the minimum allowable period of time duringwhich a particular return electrode can be disconnected. The Toffmax andToffmin values are preset prior to the start of the procedure eitherautomatically or manually. The “off” time periods may also be adjustedfor each individual return electrode.

I_(TOTAL) is the total current calculates in step 118, I_(TH) is thethreshold current calculated in step 120, and I_(high) ² is the squareof the highest return electrode current value. Thus, the “off” timeperiod is expressed as a function of the difference of the maximum andminimum “off” time periods multiplied by the ratio of the square of themeasured current and the square of the difference between the totalcurrent and the threshold current.

In step 128, the controller 25 then opens the corresponding switches 44,54 and 64 for the duration of Toff calculated in step 126. Thisdistributes the current load more evenly through the other returnelectrodes. This balances the current load and the thermal load acrossall of the return electrodes 14, 15 and 16.

Switches 44, 54 and 64 may be opened using pulse width modulation, whichallows for using predetermined pulses to manipulate specific switches.More specifically, a pulsed electrical control signal (e.g., from themicroprocessor 26) is used to toggle the switch 44 depending on the dutycycle of the control signal, such as when the signal is “on,” the switch44 is open and when the signal is “off” the switch 44 is closed.

Additional improvements to this algorithm include a comparison of totalreturn current (I_(TOTAL)) to the output current (I_(OUTPUT)) measuredby the output current sensor 28 to determine if there is any unintendedleakage paths. The comparison is made by taking into considerationleakage current which can be from about 0 mA to about 150 mA (e.g., IEC60601-2-2 maximum leakage standard). If I_(TOTAL) is larger thanI_(OUTPUT) by a corresponding leakage current amount then a warning isgiven to the user or a control signal issued by the microprocessor 26 toadjust the RF energy supply accordingly.

In another embodiment, the redistribution of the current load may beaccomplished by adjusting impedance of the circuit. Instead of theswitch 44, a device that adjusts impedance of the circuit (e.g.,resistor network, variable capacitor, transformer coupled load,transistor in linear region, etc.) including the current monitor 42 andthe return electrode 14 may be utilized. If an imbalanced current isdetected, then the impedance altering device which would be connected inseries with the circuit, may raise the impedance and thereby reduce thecurrent passing therethrough.

The current load determining algorithm may be also configured to measureimpedance of the return electrodes 14, 15 and 16 and control the currentflowing therethrough as a function of the measured impedance. If thereturn electrode is improperly adhered, the return electrode is going tohave increased relative impedance as compared with other returnelectrodes. As a result, the current passing through the improperlyadhered return electrode will decrease. The current monitors 43, 53 and63 are used to detect the decrease in current and determine if thereturn electrode having a lower current also corresponds to having anincreased impedance thereby confirming that the particular returnelectrode is improperly adhered and/or positioned.

In a further embodiment of the present disclosure, a system and methodare provided for determining the absolute value of the thermal effect ofthe return electrodes 14, 15 and 16. The value of the thermal effect isdetermined by measuring the probability of tissue damage using theimpedance values at the return electrodes 14, 15 and 16.

An algorithm in the microprocessor 26, described in more detail below,processes the signals from the output current sensor 28, the currentmonitors 43, 53 and 63 and the time calculators 30 and 32 in thecalculation of the probability of tissue damage. The output port of themicroprocessor 26 is in electrical communication with the comparator 34.The calculation of microprocessor 26 is compared to threshold valuesstored in the comparator 34, and if these values are exceeded, a signalis sent to generate an alarm using an alarm 27 as a warning to the user.If the threshold values are exceeded, the comparator 34 also sends apower adjustment signal to the controller 25 which signals the powersupply 22 to either adjust, e.g., reduce the RF output current, shut offthe power supply 22, or open any of the switches 44, 54 and 64 toterminate the supply of current, depending on the amount that thethreshold is exceeded.

The following description is of the formulas and calculations involvedin a method to calculate the probability of tissue damage occurringunder the return electrodes 14, the same method can be used for theother return electrodes. As previously stated, if the total currentpassing through the return electrode 14 is increased or the current dutycycle, defined by the percentage of time the generator 10 is “on” duringwhich the current is applied, is increased, heating under the electrodewill also increase.

Tissue damage may result when a heating factor of the tissue underneaththe return electrode 14 is higher than acceptable. The heating factor ofthe tissue is a measure of how much heat is dissipated in the tissue.Formula (3) provides the heating factor (it should be noted that in theformulas described in the disclosure, x represents the number of theassociated electrode):

Heating Factor=I_(Mx) ²t_(onx)   (3)

where I_(Mx) ² equals the square of the current in milliamps passingthrough a return electrode, e.g., I_(m14) is the current passing throughthe return electrode 14, and t_(onx) is the time that current is passingthrough a return electrode, e.g., t_(on14) time on for the returnelectrode 14. The (I_(m14)) is obtained from the corresponding currentmonitor 43 as discussed in more detail below.

Thus, the heating factor can be defined as the square of a given currentpassed through the return electrode attached to a patient multiplied bythe time the current is applied. As is apparent from the formula, ifeither the current is increased or the on time is increased, the amountof heat dissipated in the tissue, and thus the chances of tissue damage,are increased.

The foregoing heat factor formula assumes that the area attached to thepatient is unchanged. However, as a practical matter, that area canchange as a portion of the return electrode can become detached from thepatient. The return electrodes 14, 15 and 16 are split to enable theimpedance to be measured between two split pads 41 & 42, 51 & 52 and 61& 62, respectively. The impedance measurement provides an indication ofhow well the return electrodes 14, 15 and 16 are adhered to the patientsince there is a direct relationship between the impedance and the areaof patient contact. If the electrode is partially peeled from thepatient, the impedance increases. This is because each portion of theelectrode pad that touches the patient has a specific resistance. All ofthese resistances are connected in a parallel circuit, and the resultantequivalent resistance is smaller than any of its individual elements.Therefore, if any of these parallel resistances are removed because ofpeeling, the equivalent resistance increases slightly.

To accommodate for changed surface contact area of the return electrode,a constant (K_(hx)) is added to the formula where K_(hx)>=1. Forexample, K_(h14)=1 when the return electrode 14 is fully adhered, andK_(hx)>1 if the return electrode 14 is not fully adhered. Formula (4)represents the modification:

Heating Factor=K_(hx)I_(Mx) ²t_(onx)   (4)

As is apparent from the formula, if the surface contact area of thereturn electrode 14 decreases, since (K_(h14)) will be greater than 1,the heating factor will increase. As the surface area decreases, asexplained above, the current density increases and the amount of heatingfor a given output current also increases. It is to be appreciated therange of values of constant K can be determined from empirical data andstored as a database, chart, etc, which can be accessed using themeasured impedance value.

Another factor affecting dissipation of heat in the tissue is the timeperiod the RF energy is applied. The patient's body has the ability toremove heat from the area under the return electrode by the blood flowin the capillaries, small arteries and small veins. The more timebetween the applications of RF energy, the greater the heat removalbecause the body will have more time to naturally remove the heat. Thisability to remove heat over a period of time can be represented by thefollowing formula:

Cooling factor=K_(cx) t_(offx)

where (K_(c14)) is a cooling constant for the return electrode 14dependent on the patient and (t_(off14)) is the time in seconds thatcurrent is not passing through the return electrode 14.

The above-described formulas allow the method and system of the presentdisclosure to measure the current delivered and the time period thecurrent is delivered, as well as calculate and compare the heating andcooling factors to measure the probability of tissue damage as shown inFIG. 6. The method shown in FIG. 6 will be discussed with reference tothe return electrode 14 and its corresponding components.

In step 200, the current passing through the return electrode 14(I_(m14)) is measured by the current monitor 43. In step 202, thecurrent monitor 43 transmits the measurement to the microprocessor 26which squares the measurement, represented by (I_(m14) ²) in milliamps.In step 204, the time that the current being applied through the returnelectrode 14 (t_(on14)) is measured in seconds. The (t_(on14)) for thereturn electrode 14 is defined as the time during which the generator 10is activated and the return electrode 14 is in the circuit, e.g., theswitch 44 is closed. The (t_(on14)) is calculated by the time calculator30 based on the readings from the output current sensor 28 and theswitch 44. In step 206, the microprocessor 26 multiplies the time on(t_(on14)) by the squared current (I_(m14) ²), the formula beingrepresented by (I_(m14) ²)*(t_(on14)) to yield a first value.

In step 208, the impedance sensor 40 measures the impedance at thereturn electrode 14 which is indicative of the degree of adherence ofthe return electrode 14 to the patient. In step, 210 the adherenceconstant (K_(h14)) is calculated. In step 212, the microprocessor 26multiplies the adherence constant (K_(h14)) by (I_(m14) ²)*(t_(on14)) tocalculate the heating factor in step 214. Thus, the heating factor iscalculated by the algorithm which multiplies (K_(h14)) by (I_(m14)²)*(t_(on14)) wherein (K_(h14)) is the adherence constant and K=1 whenthe return electrode is fully adhered to the patient and K>1 if theelectrode is not fully adhered.

The cooling factor is calculated by the measured time the current is notbeing applied. More specifically, in step 216, the time “off” for thereturn electrode 14 in seconds of the output current (t_(off14)) iscalculated. The (t_(off14)) for the return electrode 14 is defined astime during which the generator 10 is deactivated or when the returnelectrode 14 is not in the circuit, e.g., the switch 44 is open. The(t_(off14)) is calculated by the time calculator 32 based on thereadings from the output current sensor 28 and the switch 44. In step218, the microprocessor 26 multiplies the time off (t_(off14)) by thecooling constant (K_(c14)) to calculate the cooling factor as(K_(c14))*(t_(off14)) in step 220. The cooling constant (K_(c14)) takesinto account the patient body's natural cooling where the blood flow inthe capillaries, small arteries and veins of the patient cools thetissue over time. For example, assuming tissue normally cools at onedegree per minute, since there is some variation, the cooling constantcould be conservatively selected as ½ degree per minute. Other constantscould be selected depending on the tissue cooling time.

In step 222, the cooling factor is subtracted from the heating factor bythe microprocessor 26 to determine a difference value representative ofthe probability of tissue damage. In step 224, the microprocessor 26sends a signal to the comparator 34 representative of the differencevalue and the comparator 34 compares the difference value to a firstthreshold value. If the difference value is less than or equal to thefirst threshold value, a signal sent to the controller 25 and to thepower supply 22 maintains the RF output current in step 226. Thisindicates that the differential between the cooling factor and heatingfactor is relatively low, hence there is a low probability of tissuedamage and no adjustments to the current passing through the returnelectrode 14 need to be made.

If the difference value exceeds the first threshold value, in step 228,the difference value is then compared by the comparator 34 to a secondthreshold predetermined value in step 228. The second threshold value ispreset to correspond to the situation where tissue damage is highlylikely and the RF current through the tissue needs to be terminated. Ifthe difference value exceeds the second threshold value, this indicatesthat the heating factor is too high relative to the cooling factor. Instep 232, the comparator 34 will transmit a second signal to thecontroller 25. The controller 25 will process this signal and generate ashut off signal to the power supply 22 to shut off the RF current or tothe switch 44 to turn off the current passing only through the returnelectrode 14. This shut off will allow the body time to dissipate theheat and cool the tissue.

Both threshold values are predetermined based on the probability oftissue damage so the overheating of tissue can be timely detected andthe electrosurgical generator adjusted accordingly. If the differencevalue exceeds the first threshold value, but does not exceed the secondthreshold value, this means that although the heating factor isrelatively high and there is some probability of tissue damage at thepresent power levels, it is not high enough that a shut down ismandated. Instead, the output level needs to be reduced. In thiscircumstance, in step 230, the comparator 34 will transmit a thirdsignal to the controller 25 indicative of the high probability of tissuedamage. The controller 25, in turn, will transmit a signal to the powersupply 22 or to the switch 44 to reduce the output power to therebyreduce the output current by a preset amount.

It is also contemplated that if the difference value falls between thefirst threshold value and the second threshold value, rather thanreducing the power, the duty cycle can be reduced. The duty cyclereduction could also alternately be the first response if theprobability of tissue damage exceeds a first threshold followed by areduction in power if the first threshold is further exceeded.

Thus, the system 1 remains operational, but at reduced current levels,to reduce the heating effect on the tissue. The probability of tissuedamage is preferably continuously calculated in this manner throughoutthe surgical procedure to continuously monitor and control the heatingof tissue.

As indicated in FIGS. 2 and 4, if the probability of tissue damageexceeds the first threshold value an alarm signal is sent to the alarm27 to generate an alarm. The alarm can be in the form of a visualindicator, an audible indicator or both. Additionally, a visual and/oraudible alarm can be sounded if the probability of tissue damage exceedsthe second threshold value indicating shut off of the power supply.

In an alternate embodiment, the system and method according to thepresent disclosure include an additional step of determining the size ofthe return electrode to be utilized, e.g. adult, infant, neonate, andadjusting the heating and cooling constants accordingly. The user couldinform the generator of the size being used, or alternatively, the sizecan be automatically sensed by the generator based on the differences inthe return electrode connector.

The system and method according to the present disclosure monitors thecurrent, calculates the probability of tissue damage for each of themultiple return electrodes, and adjusts the current passing through themultiple return electrodes accordingly. Since conventional returnelectrodes are connected in parallel, it is very difficult to calculatethose values using the total current output. The system according to thepresent disclosure overcomes this difficulty by using individual currentmonitors and impedance sensors for each of the multiple returnelectrodes. These devices report the current and the impedance values ofeach of the return electrode circuits. Using current values as part ofthe heating factor calculation is believed to increase the accuracy ofthe probability of a tissue damage determination since current valuesare believed to actually cause the heating of the tissue. These valuesallow the electrosurgical system to prevent tissue damage by divertingcurrent or completely turning current off and balancing the thermaleffect over multiple return electrodes. This feature, in turn, allowsfor more energy to be applied during the procedure as a whole as well asincreases the length of the surgical procedure.

While several embodiments of the disclosure have been shown in thedrawings, it is not intended that the disclosure be limited thereto, asit is intended that the disclosure be as broad in scope as the art willallow and that the specification be read likewise. Therefore, the abovedescription should not be construed as limiting, but merely asexemplifications of preferred embodiments. Those skilled in the art willenvision other modifications within the scope and spirit of the claimsappended hereto.

1. A system for determining probability of tissue damage, the systemcomprising: an electrosurgical generator adapted to generate anelectrosurgical current; a plurality of return electrodes adhered to apatient and adapted to couple to the electrosurgical generator, whereineach of the return electrodes includes an impedance sensor operativelyassociated therewith; a current monitor connected in series with each ofthe plurality of the return electrodes which measures theelectrosurgical current passing therethrough; and a processor coupled toeach of the current monitors, the processor configured to calculate acooling factor and a heating factor with respect to each of theplurality of return electrodes, the processor further configured todetermine probability of tissue damage for each of the plurality of thereturn electrodes as a function of at least one of the cooling factorand the heating factor.
 2. A system according to claim 1, wherein theprocessor is further configured to calculate the heating factor as afunction of the measured electrosurgical current passing through thecorresponding return electrode.
 3. A system according to claim 2,wherein the processor is further configured to calculate the heatingfactor as a function of measured impedance of the corresponding returnelectrode.
 4. A system according to claim 3, wherein the processor isfurther configured to calculate the heating factor as a function of anadherence constant representative of the adherence of the correspondingreturn electrode.
 5. A system according to claim 4, wherein theprocessor is further configured to calculate the heating factor as afunction of on time of the corresponding return electrode, wherein theon time is a time period during which the corresponding return electrodeis in electrical communication with the electrosurgical generator.
 6. Asystem according to claim 1, wherein the processor is further configuredto calculate the cooling factor of as a function of off time of thecorresponding return electrode, wherein the off time is a time periodduring which limited or no electrosurgical current flows through thecorresponding return electrode.
 7. A system according to claim 1,wherein the processor is further configured to calculate the coolingfactor as a function of a constant indicative of the body's ability toremove heat.
 8. A system according to claim 1, wherein the processor isfurther configured to determine the probability of tissue damage bycomparing a difference between the cooling factor and the heating factorwith a predetermined threshold.
 9. A system according to claim 1,wherein if the difference between the cooling factor and the heatingfactor exceeds the predetermined threshold the processor is configuredto perform an action selected from the group consisting of generating analarm, signaling the electrosurgical generator to adjust output of theelectrosurgical current, and signaling the electrosurgical generator toterminate output of the electrosurgical current.
 10. A method fordetermining probability of tissue damage, the method comprising:providing a plurality of return electrodes adhered to a patient andadapted to couple to an electrosurgical generator that is configured togenerate an electrosurgical current, wherein each of the returnelectrodes includes an impedance sensor and a current monitor connectedin series with each of the plurality of the return electrodes; measuringthe electrosurgical current passing through each of a plurality of thereturn electrodes; measuring the impedance of each of the plurality ofthe return electrodes; calculating a heating factor adjacent the returnelectrode for each of the plurality of the return electrodes;calculating a cooling factor adjacent the return electrode for each ofthe plurality of the return electrodes; and determining probability oftissue damage for each of the plurality of the return electrodes as afunction of at least one of the cooling factor and the heating factor.11. A method according to claim 10, wherein the step of calculating theheating factor further comprises the step of calculating the heatingfactor as a function of the measured electrosurgical current passingthrough the corresponding return electrode.
 12. A method according toclaim 11, wherein the step of calculating the heating factor furthercomprises the step of calculating the heating factor as a function ofmeasured impedance of the corresponding return electrode.
 13. A methodaccording to claim 12, wherein the step of calculating the heatingfactor further comprises the step of calculating the heating factor as afunction of an adherence constant representative of adherence of thecorresponding return electrode.
 14. A method according to claim 13,wherein the step of calculating the heating factor further comprises thestep of calculating the heating factor as a function of on time of thecorresponding return electrode, wherein the on time is a time periodduring which the corresponding return electrode is in electricalcommunication with the electrosurgical generator.
 15. A method accordingto claim 10, wherein the step of calculating the cooling factor furthercomprises the step of calculating the cooling factor of as a function ofoff time of the corresponding return electrode, wherein the off time isa time period during which no electrosurgical current flows through thecorresponding return electrode.
 16. A method according to claim 10,wherein the step of calculating the cooling factor further comprises thestep of calculating the cooling factor as a function of a constantindicative of the body's ability to remove heat.
 17. A method accordingto claim 10, wherein the step of determining probability of tissuedamage further comprises the step of comparing a difference between thecooling factor and the heating factor with a predetermined threshold.18. A method according to claim 10, further comprising the step of:performing an action selected from the group consisting of generating analarm, signaling the electrosurgical generator to adjust output of theelectrosurgical current, and signaling the electrosurgical generator toterminate output of the electrosurgical current if the differencebetween the cooling factor and the heating factor exceeds thepredetermined threshold.